It is known to provide a motor vehicle having a two wheel drive mode of operation in which motive power is supplied to one pair of wheels of the vehicle and a four wheel drive mode of operation in which motive power is supplied to each of two pairs of wheels of the vehicle. Motive power is supplied to the wheels by means of a driveline.
Some known vehicles are arranged such that motive power is permanently supplied to both pairs of wheels. Some other vehicles are arranged such that motive power is selectively supplied to either only one pair or to both pairs of wheels. A driver operable selector may be provided to allow the driver to select the two wheel drive or the four wheel drive mode of operation.
Some driveline systems require the vehicle to be stationary when transitioning between the two and four wheel drive modes. Such systems may be referred to as static disconnect/reconnect systems.
GB2407804 discloses a dynamic driveline reconnect arrangement in which reconnection of two of the wheels to the driveline following disconnection of the wheels from the driveline may be undertaken when the vehicle is moving. Such a system may be referred to as a dynamic driveline reconnect system. The system disclosed in GB2407804 employs clutch arrangements to enable dynamic driveline reconnection.
In some known dynamic driveline reconnect systems the vehicle is operable automatically to assume the four wheel drive mode when a prescribed condition is met and automatically to assume the two wheel drive mode when the prescribed condition is not met.
The present inventors have recognised that a problem exists with vehicles equipped with such systems. That is, rapid switching between the two wheel and four wheel drive conditions (a phenomenon that may be described as ‘mode chattering’) may occur when the condition for which the four wheel drive mode should be assumed is met shortly after the condition is not met.
It is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known dynamic driveline systems.